Process for preparing a 2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-1,3-propanediol derivative

ABSTRACT

Process for preparing the L-monovaline ester of 2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-1,3-propanediol and its pharmaceutically acceptable salts. The present process provides an N,O-bis-trityl intermediate of 2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-1,3-propanediol which allows for mono-esterification by an L-valine derivative. These products are of value as antiviral agents with improved absorption.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for preparing a prodrugformulation of ganciclovir and its pharmaceutically acceptable salts.More specifically, the invention relates to a process for preparing theL-monovaline ester derived from2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-1,3-propane-diol and itspharmaceutically acceptable salts.

2. Background Information

British Patent 1523865 describes antiviral purine derivatives with anacyclic chain in the 9-position. Among those derivatives2-(2-amino-1,6-dihydro-6-oxo-1,6-dihydro-purin-9-yl)methoxy-ethanol withthe INN name acyclovir has been found to have good activity againstherpes viruses such as herpes simplex.

U.S. Pat. No. 4,355,032 discloses the compound 9-(2-hydroxy-1-hydroxymethyl-ethoxy)methyl!-guanine or2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-1,3-propanediol with theINN name ganciclovir. Ganciclovir is highly efficacious against virusesof the herpes family, for example, against herpes simplex andcytomegalovirus.

European Patent Application EP 0 375 329 discloses prodrug compoundswith the following formula ##STR1## wherein R and R¹ are independentlyselected from a hydrogen atom and an amino acyl residue providing atleast one of R and R¹ represents an amino acid acyl residue and Brepresents a group of the formulae ##STR2## in which R² represents aC₁₋₆ straight chain, C₃₋₆ branched chain or C₃₋₆ cyclic alkoxy group, ora hydroxy or amino group or a hydrogen atom and the physiologicallyacceptable salts thereof. These prodrug compounds are described ashaving advantageous bioavailability when administered the oral route,resulting in high levels of the parent compound in the body.

Example 3 (b) European Patent Application EP 0 375 329 discloses thepreparation of the bis(L-isoleucinate) ester of ganciclovir as a whitefoam. Example 4 (b) discloses the preparation of the bis(glycinate)ester of ganciclovir as a white solid. Example 5 (b) discloses thepreparation of the bis (L-valinate) ester of ganciclovir as a solid.Example 6 (b) discloses the preparation of the bis(L-alaninate) ester ofganciclovir as a syrup containing 90% of the bis ester and 10% of themonoester. The bis-esters are prepared by reacting ganciclovir with anoptionally protected amino acid or functional equivalent thereof; thereaction may be carried out in a conventional manner, for example in asolvent such as pyridine, dimethylformamide, etc., in the presence of acoupling agent such as 1,3-dicyclohexylcarbodiimide, optionally in thepresence of a catalytic base such as 4-dimethylaminopyridine. Thedescribed bis esters are non-crystalline materials which are difficultto process for the manufacture of oral pharmaceutical dosage forms.

British Patent Application No. 8829571 is the priority patentapplication for European Patent Application EP 0 375 329 and U.S. Pat.No. 5,043,339, and discloses amino acid esters of the compounds of theformula ##STR3## (wherein R represents a hydroxy or amino group or ahydrogen atom) and the physiologically acceptable salts thereof.Examples of preferred amino acids include aliphatic acids e.g.containing up to 6 carbon atoms such as glycine, alanine, valine andisoleucine. The amino acid esters include both mono and diesters. Thepreparation of the diesters is identical to the preparation in EuropeanPatent Application EP 0 375 329; however, this patent application aswell as European Patent Application EP 0 375 329 and U.S. Pat. No.5,043,339 do not disclose the preparation of monoesters, or any datasuggesting their usefulness.

Leon Colla et. al., J. Med. Chem. (1983) 26, 602-604 disclose severalwater-soluble ester derivatives of acyclovir and their salts as prodrugsof acyclovir. The authors indicate that acyclovir cannot be given as eyedrops or intramuscular injections because of its limited solubility inwater and have therefore synthesized derivatives of acyclovir which aremore water soluble than the parent compound. The authors disclose thehydrochloride salt of the glycyl ester, the hydrochloride salt of thealanyl ester, the hydrochloride salt of the β-alanyl ester, the sodiumsalt of the succinyl ester, and the azidoacetate ester. The alanylesters were prepared by conventional esterification methods, includingreacting acyclovir with the corresponding N-carboxy-protected amino acidin pyridine, in the presence of 1,3-dicyclohexylcarbodiimide and acatalytic amount of p-toluenesulfonic acid and subsequently catalytichydrogenation to give the alpha- and beta-alanyl esters as theirhydrochloride salts.

L. M. Beauchamp et. al., Antiviral Chemistry & Chemotherapy (1992), 3(3), 157-164 disclose eighteen amino acid esters of the antiherpeticdrug acyclovir and their effectiveness as prodrugs of acyclovir,evaluated in rats by measuring the urinary recovery of acyclovir. Tenprodrugs produced greater amounts of the parent drug in the urine thanacyclovir itself: the glycyl, D,L-alanyl, L-alanyl, L-2-aminobutyrate,D,L-valyl, L-valyl, DL-isoleucyl, L-isoleucyl, L-methionyl, and L-prolylester. According to the authors the L-valyl ester of acyclovir was thebest prodrug of the esters investigated. These esters were prepared bymethods similar to those employed by Colla et. al.

European Patent Publication 308 065 discloses the valine and isoleucineesters of acyclovir, preferably in the L-form, as showing a largeincrease in absorption from the gut after oral administration, whencompared with other esters and acyclovir. The amino acid esters areprepared by conventional esterification methods, including reactingacyclovir with an N-carboxy-protected amino acid or an acid halide oracid anhydride of the amino acid, in a solvent such as pyridine ordimethylformamide, optionally in the presence of a catalytic base.

PCT Patent Application WO 94/29311 discloses a process for thepreparation of amino acid esters of a nucleoside analogue, includingacyclovir and ganciclovir. This process comprises reacting a nucleosideanalogue having an esterifiable hydroxy group in its linear or cyclicether moiety, with a 2-oxa-4-aza-cycloalkane-1,3-dione of the formula##STR4## wherein R¹ may represent hydrogen, C₁₋₄ alkyl or alkenyl groupor other amino acid side chains, and R² may represent hydrogen or agroup COOR³ where R³ is a benzyl, t-butyl, fluorenylmethyl or anoptionally halo substituted linear or branched C₁₋₈ alkyl group.Preferred R¹ groups include hydrogen, methyl, iso-propyl and isobutyl,yielding respectively the glycine, alanine, valine and isoleucine estersof acyclovir or ganciclovir. Examples 1-3 of PCT Patent ApplicationWO94/29311 discloses only the condensation of acyclovir with thevaline-substituted 2-oxa-4-aza-cycloalkane-1,3-dione(Z-valine-N-carboxyanhydride) by conventional procedures. While theamino acid esters of the PCT application include both the acyclovir andganciclovir (DHPG) esters, the application does not disclose how toprepare the ganciclovir esters, much less the mono-esters ofganciclovir.

The L-monovaline ester derived from2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-1,3-propane-diol and itspharmaceutically acceptable salts are potent antiviral agents and aredescribed in U.S. patent application Ser. No. 281,893 filed Jul. 28,1994. These compounds have been found to have improved oral absorptionand low toxicity. This patent application also discloses certainprocesses for preparing these esters, different from those describedherein.

The present invention relates to an improved process whereby gancicloviris reacted with a trityl compound to provide a ganciclovir intermediatewith trityl protection at one of the aliphatic hydroxyl groups and atthe 2-amino moiety of the guanine group. This intermediate is isolatedin high yields and with high purity and allows for mono-esterificationby an L-valine derivative to produce the L-monovaline ester ofganciclovir.

SUMMARY OF THE INVENTION

This invention provides a process for preparing the compound of theformula I: ##STR5## and the pharmaceutically acceptable salts thereof,which compound is named hereinafter2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-hydroxy-1-propanyl-L-valinateor mono-L-valine ganciclovir.

This process involves the reaction of ganciclovir with a trityl compoundto provide a ganciclovir intermediate with trityl protection at one ofthe aliphatic hydroxyl groups and at the 2-amino moiety of the guaninegroup of ganciclovir. This intermediate, in turn, allows formono-esterification by an L-valine derivative, to provide anN,O-bistrityl-monovaline ester of ganciclovir, followed by removal ofthe protecting groups to yield the prodrug of Formula I. Optionally, theprocess can also include the formation of salts of the prodrug ofFormula I, the conversion of an acid addition salt of the prodrug ofFormula I into a non-salt form, the optical resolution of a prodrug ofFormula I or the preparation of the prodrugs of Formula I in crystallineform. Details of the process are described below.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

Unless otherwise stated, the following terms used in the specificationand claims have the meanings given below:

"Alkyl" means a straight or branched saturated hydrocarbon radicalhaving from one to the number of carbon atoms designated. For example,C₁₋₇ alkyl is alkyl having at least one but no more than seven carbonatoms, e.g. methyl, ethyl, i-propyl, n-propyl, n-butyl,n-pentyl,n-heptyl and the like.

"Lower alkyl" means an alkyl of one to six carbon atoms.

"Aryl" means an organic radical derived from an aromatic hydrocarbon bythe removal of one hydrogen atom. Preferred aryl radicals have six totwelve carbon atoms as ring carbon atoms in the aromatic hydrocarbon.

"Aralkyl" means an organic radical derived from an aralkane in which analkyl hydrogen atom is substituted by an above-defined aryl group.

"Acyl" means an organic radical derived from an organic acid by theremoval of the hydroxyl group; e.g., CH₃ CO-- is the acyl radical of CH₃COOH, or acetyl. Other examples for such acyl groups are propionyl, orbenzoyl, etc. The term "acyl" includes the term "alkanoyl" which is theorganic radical RCO-- in which R is an alkyl group as defined above.

"Lower alkoxy", "(lower alkyl)amino", "di(lower alkyl)amino", "(loweralkanoyl)amino", and similar terms mean alkoxy, alkylamino,dialkylamino, alkanoylamino, etc. in which the or each alkyl radical isa "lower alkyl" as described above.

"Halogen" or "halo" means fluorine, chlorine, bromine, or iodine.

"Derivative" of a compound means a compound obtainable from the originalcompound by a simple chemical process. "Activated derivative" of acompound means a reactive form of the original compound which rendersthe compound active in a desired chemical reaction, in which theoriginal compound is only moderately reactive or non-reactive.Activation is achieved by formation of a derivative or a chemicalgrouping within the molecule with a higher free energy content than thatof the original compound, which renders the activated form moresusceptible to react with another reagent. In the context of the presentinvention activation of the carboxy group is of particular importanceand corresponding activating agents or groupings which activate thecarboxy group are described in more detail below. An example of anactivated derivative of L-valine is the compound of Formula III ##STR6##wherein P³ is an amino-protecting group and A is a carboxy-activatinggroup, for example, halo, a lower acyloxy group, a carbodiimide group,such as 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDAC), anisobutyrate group, and the like.

Of particular interest for the present invention is an amino acidanhydride which is an activated form of an amino acid which renders theamino acid (especially L-valine) susceptible to esterification. Aminoacid anhydrides are included in the compounds of Formula III, above.Especially useful for the present invention are the cyclic amino acidanhydrides of L-valine, described in PCT Patent Application WO 94/29311,such as 2-oxa-4-aza-5-isopropyl-cycloalkane-1,3-dione of formula IIIa:##STR7## in which P³ is an amino protecting group. Other examples of thecyclic amino acid anhydrides are protected amino acid N-carboxyanhydrides (NCA's) described in more detail below.

"Protecting group" means a chemical group that (a) preserves a reactivegroup from participating in an undesirable chemical reaction; and (b)can be easily removed after protection of the reactive group is nolonger required. For example, the benzyl group is a protecting group fora primary hydroxyl function.

"Amino-protecting group" means a protecting group that preserves areactive amino group that otherwise would be modified by certainchemical reactions. The definition includes the trityl group --C(Ph)₃(i.e., triphenylmethyl), or substituted trityl groups, such as themonomethoxytrityl group, dimethoxytrityl groups such as the4,4'-dimethoxytrityl or 4,4'-dimethoxytriphenylmethyl group, the formylgroup or lower alkanoyl groups with 2 to 4 carbon atoms, in particularthe acetyl or propionyl group, the trichloroacetyl group, thetrifluoroacetyl group, and the N-(9-fluorenylmethoxycarbonyl) or "EMOC"group, the allyloxycarbonyl group or other protecting groups derivedfrom halocarbonates such as (C₆ -C₁₂)aryl lower alkyl carbonates (suchas the N-benzyloxycarbonyl group derived from benzylchlorocarbonate), orderived from biphenylalkyl halo carbonates, or tertiary alkyl halocarbonates, such as tertiary butylhalocarbonates, in particular tertiarybutylchloro-carbonate, or di(lower)alkyldicarbonates, in particulardi(t-butyl)dicarbonate, the phthalyl group and triphenylmethyl halidessuch as triphenylmethyl chloride, and trifluoroacetic anhydride.

In the context of the present invention, an amino-protecting group maybe present at the 2-amino moiety of the guanine group of ganciclovir, orat the amino group of the L-valine derivative. Of particular interestfor the present invention are the trityl and substituted trityl groupsas amino-protecting groups for the 2-amino moiety of the guanine groupof ganciclovir.

"Trt" is a trityl or substituted trityl group, as defined above.

"Hydroxy-protecting group" means a protecting group that preserves ahydroxy group that otherwise would be modified by certain chemicalreactions. In the context of the present invention, the hydroxyprotecting group is a trityl group --C(Ph)₃ (i.e., triphenylmethyl), ora substituted trityl group, as defined above.

The trityl protecting groups, as amino- or hydroxy-protecting groups,are derived from the reaction of an optionally substituted tritylcompound with the 2-amino group on the guanine group of ganciclovir orwith the side-chain hydroxy moiety of ganciclovir. Trityl compounds arewell-known in the art; generally they are of the formula XC(Ph)₃,wherein X is chloro or bromo. Substituted trityl compounds will yieldthe substituted trityl groups as defined above, such as themonomethoxytrityl group, dimethoxytrityl groups such as the4,4'-dimethoxytrityl or 4,4'-dimethoxytriphenylmethyl group.

"Leaving group" means a labile group that is replaced in a chemicalreaction by another group. Examples of leaving groups are halogen, theoptionally substituted benzyloxy group, the isopropyloxy group, themesyloxy group, the tosyloxy group or the acyloxy group.

All the activating and protecting agents employ the preparation of thecompound of Formula I must meet the following qualifications: (1) theirintroduction should proceed quantitatively and without racemization ofthe L-valine component; (2),the protecting group present during thedesired reaction should be stable to the reaction conditions to beemployed; and (3) the group must be readily removed under conditions inwhich the ester bond is stable and under which racemization of theL-valine component of the ester does not occur.

The process of the invention may also include the optical resolution ofa prodrug of Formula I. Terminology relating to the stereochemistry andoptical resolution of these compounds is described in U.S. patentapplication Ser. No. 281,893, incorporated herein by reference. Ser. No.281,893 has been abandoned in favor of Continuation application Ser.Nos. 08/812,990 and 08/812,991.

"Optional" or "optionally" means that a described event or circumstancemay or may not occur, and that the description includes instances wheresaid event or circumstance occurs and instances in which it does not.For example, "optionally substituted phenyl" means that the phenyl mayor may not be substituted and that the description includes bothunsubstituted phenyl and phenyl wherein there is substitution;"optionally followed by converting the free base to the acid additionsalt" means that said conversion may or may not be carried out in orderfor the process described to fall within the invention, and theinvention includes those processes wherein the free base is converted tothe acid addition salt and those processes in which it is not.

"Pharmaceutically acceptable" means that which is useful in preparing apharmaceutical composition that is generally safe and non-toxic andincludes that which is acceptable for veterinary use as well as humanpharmaceutical use.

"Pharmaceutically acceptable salts" means salts which possess thedesired pharmacological activity and which are neither biologically norotherwise undesirable. Such salts include acid addition salts formedwith inorganic acids such as hydrochloric acid, hydrobromic acid,sulfuric acid, nitric acid, phosphoric acid, and the like; or withorganic acids such as acetic acid, propionic acid, hexanoic acid,heptanoic acid, cyclopentane-propionic acid, glycolic acid, pyruvicacid, lactic acid, malonic acid, succinic acid, malic acid, maleic acid,fumaric acid, tartaric acid, citric acid, benzoic acid,o-(4-hydroxy-benzoyl)-benzoic acid, cinnamic acid, mandelic acid,methanesulfonic acid, ethanesulfonic acid, 1,2-ethane-disulfonic acid,2-hydroxyethane-sulfonic acid, benzenesulfonic acid,p-chlorobenzenesulfonic acid, 2-naphthalenesulfonic acid,p-toluenesulfonic acid, camphorsulfonic acid, 4-methyl-bicyclo2.2.2!oct-2-1-ene-1-carboxylic acid, gluco-heptonic acid,4,4'-methylenebis(3-hydroxy-2-naphthoic) acid, 3-phenylpropionic acid,trimethyl-acetic acid, tertiary butylacetic acid, lauryl sulfuric acid,gluconic acid, glutamic acid, hydroxy-naphthoic acids, salicylic acid,stearic acid, muconic acid, and the like. Preferred pharmaceuticallyacceptable salts are those formed with hydrochloric, sulfuric,phosphoric acid, acetic or methanesulfonic acid, ethanesulfonic acid,1,2-ethanedisulfonic acid, 2-hydroxyethanesulfonic acid,benzene-sulfonic acid, p-chlorobenzenesulfonic acid, and2-naphthalenesulfonic acid, p-toluenesulfonic acid, camphorsulfonicacid.

Synthetic Reaction Parameters

Unless specified to the contrary, the reactions described herein takeplace at atmospheric pressure within a temperature range from 5° C. to170° C. (preferably from 10° C. to 50° C.; most preferably at "room" or"ambient" temperature, e.g., 20°-30° C.). However, there are clearlysome reactions where the temperature range used in the chemical reactionwill be above or below these temperature ranges. Further, unlessotherwise specified, the reaction times and conditions are intended tobe approximate, e.g., taking place at about atmospheric pressure withina temperature range of about 5° C. to about 100° C. (preferably fromabout 10° C. to about 50° C.; most preferably about 20° C. over a periodof about 1 to about 100 hours (preferably about 5 to 60 hours).Parameters given in the Examples are intended to be specific, notapproximate.

Isolation and purification of the compounds and intermediates describedherein can be effected, if desired, by any suitable separation orpurification procedure such as, for example, filtration, extraction,crystallization, column chromatography, thin-layer chromatography orthick-layer chromatography, or a combination of these procedures.Specific illustrations of suitable separation and isolation procedurescan be had by reference to the examples hereinbelow. However, otherequivalent separation or isolation procedures can, of course, also beused.

Presently Preferred Embodiments

While the broadest definition of this invention is set forth in theSummary of the Invention as a process for preparing the compound ofFormula I and its pharmaceutically acceptable salts, the (R,S) mixtureand certain salts are preferred.

The following acids are preferred to form pharmaceutically acceptablesalts with the compound of Formula I: hydrochloric, sulfuric, phosphoricacid, acetic, methanesulfonic, ethanesulfonic, 1,2-ethanedisulfonic,2-hydroxyethanesulfonic, benzenesulfonic, p-chlorobenzenesulfonic,2-naphthalenesulfonic, p-toluenesulfonic and camphorsulfonic acid. Mostpreferred are strong inorganic acids, such as hydrochloric, sulfuric orphosphoric acid.

The most preferred compounds are2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl) methoxy-3-hydroxy-1-propanylL-valinate hydrochloride and acetate. These compounds can be prepared ascrystalline materials and therefore can be easily manufactured intostable oral formulations.

In any of the processes described herein, a reference to Formula I, II,III, IIIa, IV or V refers to such Formulae wherein p¹, p², P³, Trt and Aare as defined in their broadest definitions set forth in the Summary ofthe Invention, with the processes applying particularly to the presentlypreferred embodiments.

Details of the Synthetic Processes

The process of the present invention is depicted in the ReactionSequence shown below: ##STR8## wherein P¹ is hydrogen or anamino-protecting group, P² is an optionally substituted trityl group oran amino-protecting group, P³ is an amino-protecting group and Trt is anoptionally substituted trityl group.

2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-1,3-propanediol(ganciclovir) is reacted with an optionally substituted trityl compoundas defined above to yield the N,O-bis trityl ganciclovir intermediatewith trityl protection at one of the aliphatic hydroxy groups and at the2-amino function (Formula II). The product of this reaction isesterified with an L-valine derivative of Formula III or IIIa to form anN,O-bistrityl-monovaline ester of ganciclovir (Formula IV), followed byremoval of the trityl protecting groups, and any other protecting groupsto afford a compound of Formula I.

This process of this invention provides a quick and easy method forpreparing the N,O-bistrityl ganciclovir intermediate of Formula II. Thebis-trityl intermediate can be recovered with high purity and in yieldsof greater than 60%, and can be easily separated from the mono- andtritritylated impurities by filtration and simple extraction. Thisintermediate then allows for mono-esterification with an L-valinederivative to provide the compounds of Formula I.

Compounds of Formula I can optionally be converted into apharmaceutically acceptable salt thereof. The process can also includethe conversion of an acid addition salt of the prodrug of Formula I intoa non-salt form, the optical resolution of a compound of Formula I orthe preparation of the compound of Formula I in crystalline form.

The process for producing the compound of the Formula I may or may notinvolve protection of the amino group in the 2-position of the guaninebase, though the preferred process is carried out without a protectedamino group. For the case when the ganciclovir starting material doeshave a protected 2-amino group, the protecting group may be removed byconventional procedures, well-known in the art. For example, if theamino-protecting group is a lower alkanoyl group basic conditions (pHbetween 8 to 11) are employed to remove the protecting group. Forexample, 2-N-acetyl ganciclovir is treated with an alkaline reagent suchas ammonium hydroxide, sodium or potassium carbonate or sodium orpotassium hydroxide until the removal of the acetyl group is complete.In general, this reaction will be conducted in the presence of asuitable solvent such as a lower alkanol. Preferably the startingmaterial is dissolved in methanol and a stoichiometric excess ofammonium hydroxide is added. The reaction temperature is kept between 0°to 50° C., preferably at room temperature. After the reaction iscomplete (which can be determined by TLC), another solvent may be addedto facilitate isolation of the de-protected product, such as ethyl etherwhich leads to precipitation of the de-acylated product which can befiltered off and isolated using conventional separation methods.

Starting Materials

All starting materials employed to make the compound of Formula I areknown, such as ganciclovir, the trityl compounds, and the protecting andcarboxylic-group-activating reagents.

Prior to carrying out Step II (esterification step), the amino group ofthe L-valine derivative must be protected to avoid its interference withthe esterification by undesirable amide formation. The variousamino-protected L-valine derivatives useful in this invention, such asN-benzyloxycarbonyl-L-valine, BOC-L-valine and FMOC-L-valine,N-formyl-L-valine and N-benzyloxycarbonyl-N-carboxy-L-valine anhydride,are all commercially available (SNPE Inc., Princeton, N.J., AldrichChemical Co., Milwaukee, Wis. and Sigma Chemical Co., St. Louis, Mo.),or are described in the literature, such as N-allyloxycarbonyl-L-valine.Cyclic amino-protected L-valine derivatives are also described in theliterature, as noted above. Of particular interest for the presentinvention is the benzyloxycarbonyl valine-substituted2-oxa-4-aza-cycloalkane-1,3-dione (Z-valine-N-carboxyanhydride, orZ-Valine-NCA), and Boc-L-valine, which are also commercially available(SNPE Inc., Princeton, N.J.). Alternatively, the protecting step may becarried out by conventional methods.

A preferred ganciclovir starting material for the preparation of thecompound of the invention is the unprotected ganciclovir(2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-1,3-propanediol) whichis described in U.S. Pat. No. 4,355,032. Other ganciclovir startingmaterials may have protection at the 2-amino group, such as2-(2-acyl-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-1,3-propanediol

Preferred trityl compounds are trityl chloride and trityl bromide.

Preparation of Activated derivative of L-valine

Prior to carrying out Step II (esterification step), L-valine must alsobe activated. At least 1 equivalent of the protected amino acid and 1equivalent of a suitable coupling agent or dehydrating agent, forexample 1,3-dicyclohexylcarbodiimide or salts of such diimides withbasic groups should be employed from the start. Other carbodiimides suchas N,N'-carbonyldiimidazole may also be used. Further useful dehydratingagents are trifluoroacetic anhydride, mixed anhydrides, acid chlorides,1-benzo-triazolyloxy-tris (dimethylamino)phosphoniumhexafluorophosphate, benzotriazole-1-yl-oxy-trispyrrolidinophosphoniumhexafluorophosphate, 1-hydroxybenzotriazole,1-hydroxy-4-azabenzotriazole, 1-hydroxy-7-azabenzotriazole,N-ethyl-N'-(3-(dimethylamino)-propyl) carbodiimide hydrochloride,3-hydroxy-3,4-dihydro-4-1,2,3-benzotriazine,O-(7-azabenzotriazol-1-yl)-1,1,3,3-tetramethyluroniumhexafluorophosphate,O-(7-azabenzotriazol-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate,O-(1H-benzotriazol-1-yl)-1,1,3,3-bis(tetramethylene)-uroniumhexafluorophosphate orO-(7-azabenzotriazol-1-yl)-1,1,3,3-bis-(tetramethylene)uroniumhexafluorophosphate. A description of these coupling agents by L. A.Carpino can be found in J. Chem. Soc. 1993, 115, p. 4397-4398.

Also useful for this purpose are urethane-protected amino acid N-carboxyanhydrides (UNCA's) which are an activated form of an amino acid; thesehave been described by William D. Fuller et.al., J. Am. Chem. Soc. 1990,112, 7414-7416, which is incorporated herein by reference. Otherprotected amino acid N-carboxy anhydrides are described in PCT PatentApplication WO 94/29311 discussed above. In summary, any other reagentthat produces an anhydride or another activated derivative of theprotected amino acid under mild conditions can be used as the couplingagent.

The amino-protected amino acid is dissolved in an inert solvent such asa halogenated lower alkane, preferably dichloromethane under an inertatmosphere, for example nitrogen, and the coupling agent is added(preferably 1,3-dicyclohexylcarbodiimide). The reaction mixture isstirred at temperatures between 0° and 50° C. preferably at about roomtemperature. The reaction mixture is filtered and the reaction product(the anhydride of the protected amino acid) isolated. The resultingproduct is dissolved in a dry inert solvent such as drydimethylformamide and placed under nitrogen.

Preparation of Mono-L-valine Ganciclovir

Step I:

Ganciclovir is reacted with a trityl compound to give the ganciclovirintermediate with trityl protection at one of the aliphatic hydroxygroups and the 2-amino group of the guanine ring system of ganciclovir(Formula II).

For example, a solution of 2-3 equivalents of trityl chloride is addedto a suspension of ganciclovir in an inert solvent, preferably anaprotic polar solvent, such as dimethylformamide, at a temperature of0°-100° C. preferably at 400°-60° C. This yields a mixture ofN-trityl-ganciclovir, O-trityl-ganciclovir, N,O-bistrityl-ganciclovirand tris-trityl-ganciclovir. The tris-trityl-ganciclovir is insolubleand is filtered off; the mono- and bis-tritylated-ganciclovir areprecipitated from the solvent solution, which provides some purificationof bis-tritylated compound. Further purification is achieved by washingwith an organic solvent such as ethyl acetate, and any bis-trityldissolved may be recovered by crystallization from this ethyl acetatewash.

Step II:

In this step an activated derivative of amino-protected L-valine of theFormula III or IIIa is esterified with the N,O-bistrityl-ganciclovirintermediate obtained in Step I. Suitable amino-protecting groups forthe L-valine derivative are the N-benzyloxycarbonyl group, the phthalylgroup, the tertiary butyloxycarbonyl group and theN-(9-fluorenylmethoxycarbonyl) or "FMOC" group.

A solution of the N,O-bistrityl-ganciclovir product of Step I in aninert solvent is added to about 1-3 equivalents of the activatedL-valine derivative, preferably Z-valine-N-carboxyanhydride orBoc-L-valine, in the presence of an organic base, preferably triethylamine (TEA). The reaction mixture is stirred at 10°-40° C., preferablyat ambient temperature for 10-90 hours, preferably about 24 hours. Theproduct, such as N,O-bistrityl-O-(N.sup.α (Nα-Z-L-Valine)ganciclovir iscollected by filtration, washed and dried under vacuum at 10°-70 ° C.

Step III:

The trityl protecting groups can be removed after completion of theprevious steps by dissolving the product of Step II in a polar organicsolvent such as trifluoroethanol (TFE), followed by the addition ofanhydrous alkanoic acid such as trifluoroacetic acid (TFA). The reactionmixture is stirred for 30 minutes to 10 hours at ambient temperatureuntil the reaction is complete by TLC. Solid material is removed byfiltration. Most of the trifluoroethanol/trifluoroacetic acid is removedby distillation. The reaction mixture is then slowly added to a mixtureof organic solvents, preferably tertbutyl methyl ether (MTBE):hexane(1:1). The product precipitates out of solution and is collected byfiltration, washed and dried under vacuum 10-24 hours. The product canbe further purified by crystallization.

Step IV (Final De-protection to Give the Product of Formula I)

The valine protecting group of the product of Step III is removed by ade-protection reaction, preferably in an acidic medium or solvent, mostpreferably by hydrogenolysis. De-protection under acidic conditions ispreferred, as this will ensure that the amino group liberated in thede-protection reaction will be protonated; that is, that the base ofFormula I as it is formed in the de-protection reaction will be capturedby an at least stoichiometric amount of acid present. Isolating thecompound of Formula I as an acid addition salt will protect the desiredstereoconfiguration of the compound of Formula I. Therefore, thoseexamples given below at show the de-protection step also show theconcomitant salt formation step.

The de-protection reaction is carried by dissolving the product of theprevious step in an inert solvent, preferably in an acidic solvent,using a hydrogenation catalyst, such as platinum, or palladium hydroxideon carbon or palladium on carbon, using elevated hydrogen pressurebetween 1 and 2000 psi, preferably 50 to 200 psi, most preferably 5 to20 psi. The completion of the reaction can be monitored usingconventional TLC analysis. The hydrogenolysis is continued until theconversion is complete, if required with addition of furtherhydrogenation catalyst. The catalyst is removed and washed. The combinedfiltrates from filtration and the washings are concentrated andlyophilized to isolate ganciclovir L-valine ester. The purification ofthe product and the isolation of a crystalline ester is carried out byrecrystallization or other purification techniques, such as liquidchromatographic techniques.

If present, any protecting group at the 2-amino group of the guaninegroup may be removed by conventional procedures, as described above.

If the tertiary butyloxycarbonyl group is being used as amino-protectinggroup, its removal is effected with acid, such as HCl and isopropanol asa solvent or with trifluoroacetic acid.

Preparation of Salts

One of ordinary skill in the art will also recognize that the compoundof Formula I may be prepared as an acid addition salt or as thecorresponding free base. If prepared as an acid addition salt, thecompound can be converted to the free base by treatment with a suitablebase such as ammonium hydroxide solution, sodium hydroxide, potassiumhydroxide or the like. However, it is important to point out that thefree base of Formula I is more difficult to characterize than its acidaddition salts. When converting the free base to an acid addition salt,the compound is reacted with a suitable organic or inorganic acid(described earlier). These reactions are effected by treatment with anat least stoichiometric amount of an appropriate acid (in case of thepreparation of an acid addition salt) or base (in case of liberation ofthe free compound of Formula I). In the salt-forming step of thisinvention, typically the free base is dissolved in a polar solvent suchas water or a lower alkanol (preferably isopropanol) and mixturesthereof and the acid is added in the required amount in water or inlower alkanol. The reaction temperature is usually kept at about 0° to50° C., preferably at about room temperature. The corresponding saltprecipitates spontaneously or can be brought out of the solution by theaddition of a less polar solvent, removal of the solvent by evaporationor in a vacuum, or by cooling the solution.

Isolation of Stereoisomers and the Manufacture of Crystalline2-(2-Amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-hydroxy-1-propanyl-L-valinate

From the Formula I it is apparent that the compound of the invention hasone asymmetric carbon atom (chiral center) in the propanyl chain, inaddition to the asymmetric carbon atom in L-valine. Therefore, twodiastereomeric forms exist, the (R)- and (S)- form as determined by therules of Cahn et al. Suitable methods for the separation of thediastereomers are described in U.S. patent application Ser. No. 281,893,incorporated herein by reference.

The compounds of Formula I may also be prepared in crystalline form,which has many well-known advantages over the non-crystalline form.Suitable methods for the preparation of the compounds of the inventionin crystalline form are also described in U.S. patent application Ser.No. 281,893, incorporated herein by reference.

The following preparations and example skilled in to enable thoseskilled in the art to more clearly understand and to practice thepresent invention. They should not be considered as limiting the scopeof the invention, but merely as being illustrative and representativethereof.

EXAMPLE 1 Preparation ofN-trityl-2-(2-Amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-trityloxy-propan-1-ol

2-(2-Amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-1,3-propanediol(ganciclovir) was bis-tritylated at the N and O functionalities asfollows: to a slurry of 4.1 kg ganciclovir, 9.5 g DMAP, and 8.65 kg TEAin dimethylformamide (38.3 kg ) at 50° C. was added a solution of 11.9kg trityl-chloride in 38.2 kg dimethylformamide over 25 minutes. Thereaction mixture was heated and stirred at 50° C. overnight. Thefollowing morning the reaction was complete by HPLC. This yields amixture of N-trityl-ganciclovir, O-trityl-ganciclovir,N,O-bis-trityl-ganciclovir, and tris-trityl-ganciclovir in a approximateratio of mono to bis to tri-substituted ganciclovir of 1:2:1. Thetris-trityl-ganciclovir which is dimethylformamide insoluble is thenfiltered from the dimethylformamide mixture by cooling the reactionmixture to 10° C. and filtering out the tris-trityl-ganciclovir. Thetris-tritylated-ganciclovir was washed with 10 kg dimethylformamide, andthe transfer lines were washed with 2.3 kg dimethylformamide. The mono-and bis-tritylated-ganciclovir were then precipitated from thisdimethylformamide solution by warming thebistrityl-ganciclovir/monotrityl-ganciclovir/dimethylformamide solutionto 20° C. and adding 90 kg water over 1 hour. An additional 60 kg waterwas then added. Some purification of the bistrityl-ganciclovir from themonotrityl-ganciclovir occurs at this point. The temperature of theabove mixture rose to 32° C. The mixture was seeded with 0.5 g ofN,O-bistrityl-ganciclovir and allowed to age overnight at roomtemperature. The following morning crystals had formed. The solid wasfiltered out. Further purification of the bis-trityl-ganciclovir wasachieved by washing the solid with 50 kg of water and then with 21 kg ofethyl acetate. The solid was dried under a stream of nitrogen overnightand then in a vacuum oven (˜50° C., ˜650 millitorr, nitrogen sweep) for4 days. Yield: 63.2%. Weight: 7.51 kg. HPLC: greater than 90%N,O-bistrityl-ganciclovir, impurities 1.8%.

EXAMPLE 2 2A. Preparation ofN-trityl-2-(2-Amino-1,6-dihydro-6-oxo-purin-9-yl)-methoxy-3-trityloxy-1-propanyl-N-(benzyloxycarbonyl)-L-valinate

N,O-bistrityl-ganciclovir was coupled with 1.2 equivalents ofZ-Valine-NCA (N.sup.α -benzyloxycarbonyl-L-Valine-N-carboxy anhydride)as follows: 3.7 kg of N,O-bistrityl-ganciclovir was dissolved in 17.7 kgdimethylformamide and 1.85 L toluene. The toluene was distilled off (29ins of Hg, to 50° C.) for 2.5 hours. Thedimethylformamide/N,O-bistrityl-ganciclovir solution was cooled to 22°C., and then 0.7 L TEA/0.2 L dimethylformamide was added. To thismixture was added a solution of 1.67 kg Z-Valine-NCA dissolved in 4.2 Ldimethylformamide over 0.5 hours at 22° C. The reaction was checked byTLC (10% methanol in methylene chloride, silica) and after 1 hour, thereaction was about 90% complete. The reaction was left mixing overnight.The following morning, the reaction was complete by HPLC. The coupledreaction mixture was added to 23 kg of water over 20 minutes, and thepot temperature rose from 22° C. to 31° C. This mixture was cooled to26° C. and aged for 45 minutes. Then the mixture was cooled to 10° C.over 2 hours and 15 minutes. The solid was filtered out and dried in avacuum oven (50° C., ˜28 ins of Hg, nitrogen sweep) for 4 days. Weight:4.8 kg. HPLC: 93.5% N,O-bistrityl-O-(N.sup.α -Z-L-valine)-ganciclovir.

2B. Preparation ofN-trityl-2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)-methoxy-3-trityloxy-1-propanyl-N-(butyloxycarbonyl)-L-valinate

The symmetric anhydride of Boc-L-Valine was formed by adding 12.6 g1,3-dicyclohexylcarbodiimide (DCC) in 80 ml dichloromethane to 26.5 gBoc-L-Valine dissolved in 100 ml dichloromethane at 0⁰⁻² ° C. Thetemperature rose to about 10° C. during the DCC addition. The solutionwas stirred for 30 minutes at 0° to 3° C. and then the1,3-dicyclohexylurea (DCU) was filtered out of solution. This solutionwas added to 36.5 g N,O-bistrityl-ganciclovir in 180 mldimethylformamide in the presence of 63.6 mg of DMAP and 8.5 ml TEA at11° C. The coupling mixture was stirred at room temperature for 16.5hours. The reaction was judged complete by HPLC (91%N,O-Bistrityl-O-(N.sup.α -Boc-L-Valine)-ganciclovir, 0.16%N,O-Bistrityl-ganciclovir.) 380 ml of water was added and the reactionmixture was stirred for 1 hour. The dichloromethane layer was separatedand the solvent was evaporated to obtain a residue. The aqueous layerwas extracted with 750 ml ethyl acetate. The ethyl acetate was separatedand then combined with the dichloromethane residue. 750 ml of toluenewas added to this ethyl acetate layer. The organic layer was washed oncewith 250 ml water, once with 250 ml 5% NaHCO₃, and twice with 250 mlwater. The organic layer was dried over MgSO₄, then the MgSO₄ filteredout. The solvents were evaporated off to 50° C. A residue was obtained.Weight: 53.64 g (some solvent still present.)

2C. Preparation ofN-trityl-2-(2-Amino-1,6-dihydro-6-O-purin-9-yl)-methoxy-3-trityloxy-1-propanyl-N-(butyloxycarbonyl)-L-valinate

To 1 g of N,O-Bistrityl-ganciclovir in 5 ml dimethylformamide was added460 mg of Boc-L-Valine-NCA, and 0.2 ml TEA (stirred overnight.) Thereaction was checked and judged complete by TLC (90%N,O-Bistrityl-O-(N-.sup.α -Boc-L-Valine)ganciclovir by HPLC.) 5 ml ofwater was added to this solution to precipitate out the product. Thesolid was filtered out and washed with water. The solid was dried in avacuum oven (about 50° C., 25 ins of Hg, nitrogen sweep) overnight.Weight: 1.12 g. HPLC: 92% N,O-Bistrityl-O-(N.sup.α-Boc-L-Valine)-ganciclovir, 3.6% N,O-Bistrityl-ganciclovir.

EXAMPLE 3 Preparation of2-(2-Amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-hydroxy-1-propanyl-N-(benzyloxycarbonyl)-L-valinate

The trityl groups of N,O-bistrityl-(N.sup.α -Z-Valine)-ganciclovir wereremoved as follows: To 4.8 kg of N,O-bistrityl-O-(Nα-Z-L-Valine)ganciclovir in 17.4 kg TFE was added a solution of 1.7 L TFA in 17.3 kgTFE over 15 minutes. The reaction temperature rose from 18° to 20°C.After 4 hours of mixing the reaction was 85% to 95% complete by TLC (10%methanol in methylene chloride, silica). After 5 hours of mixing thesolid was filtered out. The solid was washed with 5.6 kg of TFE. TFE/TFAwas vacuum distilled off (pot temperature to 45° C., vacuum to 26 ins ofHg) over 1 hour and 20 minutes. After distillation of about 80% of theTFE by volume, the pot residue was added to 37 kg MTBE and 32.4 kg ofhexane and with vigorous mixing. The precipitant was then filtered out,and the solid was washed with a solution of 7.4 kg MTBE/6.6 kg hexane.The solid was dried under a stream of nitrogen. This O-(N.sup.α-Z-L-Valine)-ganciclovir was dissolved in 38.8 kg methanol at 41° C.; 59kg of water was added while distilling off the methanol to a jackettemperature of 58° C., about 28 ins of Hg. The pot temperature rangedfrom 16° C. to 26° C. The solid was filtered out and washed with 13.2 kgwater. The solid was dried under a stream of nitrogen; 1.2% water wasdetermined by the Karl Fisher test. Weight: 1.78 kg. HPLC: greater than90% Z-Valine-ganciclovir, less than 0.5% ganciclovir.

EXAMPLE 4 Preparation of2-(2-Amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-hydroxy-1-propanyl)-L-valinatehydrochloride

4A. 10 g of N,O-Bistrityl-O-(N.sup.α -Boc-L-Valine) ganciclovir wasdissolved in 20 ml dichloromethane. To this was added 80 ml 50% TFA indichloromethane, and the reaction mixture was stirred for 1 hour.Cleavage of the trityl groups and the valine protecting groups werejudged complete by TLC. The reaction solution was added dropwise to 1 Lof (1:1) MTBE: hexane over 30 minutes. The precipitate was filtered outand was washed with 500 ml (1:1) MTBE: hexane. The solid was dried in avacuum oven (about 50° C., 25 ins of Hg, nitrogen sweep) overnight.Crude weight of precipitate: 4.31 g. The solid was dissolved in 200 mlacetone containing 2 to 3 ml acetic acid. To this solution was added 4ml of 4.2M HCl in ethyl acetate, forming a solid. This was followed bystirring for 45 minutes and then an additional ml of 4.2M HCl in ethylacetate was added. This was stirred 30 minutes and then the solid wasfiltered out. The solid was washed with 50 ml acetone and then dried ina vacuum oven (about 50° C., 25 ins of Hg, nitrogen sweep) overnight.Weight: 3.82 g, HPLC 97.4% L-Valine-ganciclovir, 2.1% ganciclovir, 0.5%Guanine.

4B. Alternatively,2-(2-Amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-hydroxy-1-propanyl)-L-valinatemay be prepared directly from the N,O-Bistrityl-O-(N.sup.α-Boc-L-Valine)-ganciclovir as follows: To 471 mg ofN,O-Bistrityl-O-(Nα-Boc-L-Valine)-ganciclovir was added 10 ml of 2.1MHCl in Bistrityl-O-(N.sup.α -Boc-L-Valine)-ganciclovir was added 10 mlof 2.1M HCl in ethyl acetate. This reaction mixture was stirred for 15minutes, and the reaction judged complete by TLC. The solid was filteredout and washed with ethyl acetate. The solid was dried in a vacuum oven(about 50° C., 25 ins of Hg, nitrogen sweep.) Weight: 165 mg, HPLC: 91%L-Valine-ganciclovir, 5.9% guanine, 2.3% ganciclovir.

EXAMPLE 5 Preparation of2-(2-Amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-hydroxy-1-propanyl-L-valinatehydrochloride

The Z (benzyloxycarbonyl) group was removed from O-(N.sup.α-Z-Valine)-ganciclovir prepared in Example 3 as follows: To 174 g of 10%Pd on carbon and 1.74 kg of Z-Valine-ganciclovir was added 14 kg ofmethanol mixed with 357 ml 12M HCl. This reaction mixture was placedunder 5 to 10 psi hydrogen. After 2.5 hours of agitation an aliquot wasremoved from this reaction mixture. The reaction was complete by TLC (10ml acetonitrile, 1 ml water, 1 ml acetic acid, silica). After 3.5 hoursof agitation the catalyst was removed by filtration of the reactionmixture through a Solka Floc bed. The Solka Floc bed was washed with13.7 kg of methanol. The methanol solutions were combined. Methanol wasremoved by rotary evaporation to a 60° C. bath temperature, and a finalvacuum of about 28 ins of Hg. The residue was then dissolved in 1.4 Lsterile water at 50° to 60° C. This solution was washed with 1 L ethylacetate, and then the ethyl acetate layer was separated. Any residualsolvent was removed from this aqueous solution by evaporation. 5.6 L ofapproximately 50° C. isopropyl alcohol was added dropwise to thisaqueous solution at 50° C. The reaction mixture was seeded with aprevious batch of2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-hydroxy-1-propanyl-L-valinatehydrochloride. This crystallization mixture was allowed to cool slowlyto 30° C., after which an additional 4.2 L of isopropyl alcohol wasadded dropwise at 37° C. to 30° C. This mixture was allowed to coolslowly to room temperature with agitation overnight. The followingmorning the crystallization mixture was cooled to -5° C., and aged for 2hours. The solid was filtered out and then washed with 2 L cold 5% waterin isopropyl alcohol and then 4 L cold isopropyl alcohol. The solid wasdried in a vacuum oven (675 millitorr, 50° C., nitrogen sweep) for 50hours. Mass of solid 1.05 kg. Overall yield from ganciclovir startingmaterial: 34%. HPLC: 98.4%2-(2-Amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-hydroxy-1-propanyl-L-valinatehydrochloride; MS: 355 (MH)⁺.

What is claimed is:
 1. A process for preparing the compound2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-hydroxy-1-propanyl-L-valinate or a pharmaceutically acceptable salt ordiastereomer thereof, comprising:(a) reacting2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-1,3-propanediol of theformula ##STR9## with a trityl compound of the formula (Ph)₃ CX, whereinPh is phenyl, optionally substituted with methoxy, and X is bromo orchloro, to produce a compound of formula II ##STR10## wherein Trt is atrityl group of the formula (Ph)₃ C- as defined above; (b) esterifyingthe compound of formula II with an activated protected derivative ofL-valine of formula III or formula IIIa ##STR11## wherein P³ is anamino-protecting group and A is a carboxy-activating group, to produce acompound of formula IV ##STR12## wherein P³ and Trt are as definedabove; and (c) deprotecting the compound of formula IV to2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-hydroxy-1-propanyl-L-valinate or a pharmaceutically acceptable saltthereof; optionally followed by (d) converting2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-hydroxy-1-propanyl-L-valinateinto a pharmaceutically acceptable salt thereof; or (e) separating the2-(2-amino-1,6-dihydro-6-oxo-purin-9-yl)methoxy-3-hydroxy-propanyl-L-valinateinto its (R) and (S) diastereomers.
 2. The process of claim 1 whereinthe trityl compound is trityl chloride or trityl bromide.
 3. The processof claim 1 wherein the activated protected derivative of L-valine is ananhydride of formula IIIa ##STR13## wherein p³ is an amino-protectinggroup.
 4. The process of claim 3 wherein the protected L-valinederivative is N-t-butyloxy-carbonyl-N-carboxy-L-valine anhydride orN-benzyloxycarbonyl-N-carboxy-L-valine anhydride.
 5. The process ofclaim 1 wherein step (c) comprises removing the trityl protecting groupsin a polar organic solvent or dichloromethane in the presence of analkanoic acid.
 6. The process of claim 5, wherein the trityl groups areremoved in trifluoroethanol or dichloromethane in the presence oftrifluoroacetic acid.
 7. The process of claim 1 wherein p³ isbenzyloxycarbonyl or t-butyloxycarbonyl.
 8. The process of claim 7wherein step (c) includes hydrogenolysis.
 9. The process of claim 8wherein the hydrogenolysis is carried out in methanol/hydrochloric acidin the presence of platinum, palladium hydroxide on carbon or palladiumon carbon.
 10. The process of claims 9 wherein P³ is benzyloxycarbonyland the hydrogenolysis is carried out in the presence of palladium oncarbon.